1. Field of the Invention.
This invention relates generally to the field of building construction and in particular to structural framing elements for building construction.
2. Description of the Prior Art.
The vast majority of buildings are wood frame construction. Wood frame buildings are subjected to many forces. Among the most significant are gravity, wind, and seismic forces. Gravity is a vertically acting force, wind and seismic forces are primarily lateral (horizontal). Many wood frame buildings use shearwall panels to resist lateral loads. A shearwall panel is formed by the application of one or more types of sheathing such as, plywood, fiberboard, particle board, and or drywall (gypsum board), to the inside or outside or both sides of a wall frame. The sheathing is fastened to the wall frame at many points creating a shearwall panel. Many suitable fasteners are available, nails are commonly used and will be referred to hereafter. The sheathed shearwall panel is used to conduct the lateral force acting on the frame of the building to the foundation.
Buildings require a strong base for support. Most buildings have a concrete base that is generally referred to as the foundation. A concrete pad whose top forms a continuous plane from edge to edge is called a slab. With a slab the concrete forms the floor of the building. The deepest concrete support that follows the perimeter of the building is called the footing. In a building without a concrete floor, the floor may be supported by short concrete walls called stem walls that are supported by the footing. Some grading considerations or design requirements necessitate a hybrid of a slab and a stem wall. This results in the use of short concrete walls extending from a few inches to a few feet above the level of the concrete floor. Foundation will be used hereafter in place of stem wall, footing, and slab.
The site where the building is to be erected is first graded (leveled). Wooden boards are nailed together to create a `form` or mold for the foundation (slab, footing, stem wall). The forms mark the edges of the foundation. Next, wet concrete is poured into the form and the surface is smoothed and the concrete is allowed to harden. As the concrete hardens, bolts are partially imbedded in the top of the foundation with the threaded end of each bolt protruding out of the foundation. The bolts are embedded wherever a wall will contact the foundation/stem wall to provide a means of securing the wall to the foundation.
The frame of the walls is fabricated next. Each wall frame section is composed of several elements. In North America, the wall frames of most homes and small buildings use boards having cross sectional dimensions of 2".times.4", 2".times.6", or 2".times.8". At the base of the wall frame is a board called the mudsill. The mudsill is usually a 2".times.4" board chemically treated to resist rotting. The studs are nailed on top of the mudsill. The studs are generally 2".times.4" boards standing on end usually 16" apart. On top of the studs is a board called the top plate that is nailed to each stud. The top plate is usually a 2".times.4" board. The wall frame is nailed together while all the parts are lying flat on the foundation. Holes are drilled through the mudsill for the foundation bolts to pass through the mudsill. After the wall frame is nailed together, the wall frame is tilted to a vertical orientation. The wall frame is put in its finished location with the foundation bolts protruding through the holes drilled in the mudsill. The wall frame is braced until the adjacent wall frames are in place. Once adjacent wall frames are in place, they are nailed together at the corners and an additional plate (top cap) is added which overlaps the top plates of adjacent wall frames.
Once the wall frames are in place the supports for the ceiling may be attached. The ceiling supports are called ceiling joists, and they rest on and are attached to the walls at the top cap. The joists are parallel to the foundation and span the distance from one wall in a room to the opposite wall in the room. After the ceiling joists are in place, the roof is framed. The roof frame members (rafters) are also attached to the top cap. In many buildings the ceiling joists and the roof framing are combined by the use of trusses. A roof truss is generally triangular and is composed of the roof rafters and the ceiling joists all prefabricated together of usually 2".times.4" boards.
After the building frame is completed, the building is ready to be sheathed. Conventional building construction uses sheathing inside a building (drywall) which forms the wall surface which we all see, and sheathing on the roof which helps keep the building dry. Plywood or other sheathing is also applied to the outside and sometimes the inside walls of every building. The panel created by many nails driven through the plywood or drywall into the supporting wall studs, mud sill and top plates creates a sturdy vertical diaphragm known as a sheathed shearwall. Drywall or gypsum sheathing provides insulation and fire resistance and some structural stability. The structural contribution of a drywall panel is limited because of the relatively delicate composition of the drywall. Where higher lateral force resistance is required, builders and designers generally use plywood or particle board or fiberboard sheathing fastened to the wall frame. Plywood is the most common choice and will be used hereafter, but other suitable materials may be used. Plywood is available in 4'.times.8' sheets that vary from 1/4" to over 1" in thickness. Plywood is composed of many thin layers of wood glued together under pressure with the grain pattern of adjacent layers perpendicular to each other for strength.
Review of damage following the Northridge earthquake, revealed that many plywood sheathed shearwalls failed under the seismic forces. The nailing of the sheathing in the field during construction leads to many failures. Nails driven through the sheathing miss the frame member they were intended to penetrate creating `shiners`. Nail heads penetrate the skin of the sheathing during nailing which weakens the sheathing and allows the nails to be pulled through the sheathing under load conditions as well as inducing failures in the integrity of the sheathing. Shearwall fabrication requires regular nail spacing of 3"-12" depending on the design requirements. Current field fabrication techniques are not sufficiently accurate to consistently meet the design specifications. Therefore every shearwall panel may be nailed differently and many may be installed with fewer nails than required to handle the required design load.
The rise in land prices has caused the building of more multiple floor dwellings to raise housing density. Multiple floors significantly increase lateral loads and thus increase the use of field fabricated sheathed shearwalls. In many multiple story buildings the entire outside of the building may be sheathed.
Consequently many of the building departments in California are limiting sheathed shearwalls to a maximum height/width ratio of 2:1. Where walls are typically eight feet high, the minimum shearwall width would be four feet. This restriction has implications throughout a building. At the front of a garage narrow shearwalls, two to three foot wide, are common. Narrow sheathed shearwalls are also common adjacent to window and door openings.
The interface between the shearwall and the foundation may also be area of weakness. The conventional practice of locating holdowns within the framework of a sheathed shearwall weakens the sheer wall and the frame-foundation interface. Bolts imbedded in the concrete of the foundation provide attachment points for the walls and shear panels. These bolts are intended to pass through the mudsill of the sheathed shearwall to prevent lateral movement between the sheathed shearwall and the foundation. The foundation bolts also transfer the lateral load from the top of the sheathed shearwall to the foundation. Quite often the bolts which are supposed to secure the walls and shear panels are placed several inches away from where they are required for optimum load transfer and ease of wall construction due to inaccurate measuring and carelessness during field installation of the bolts. The resulting misalignment forces some of the framing members to be trimmed to fit, or in some cases, the intended foundation bolt must be cut off and a "redhead" must be used. A "red head" is a bolt forced into a hole drilled into the foundation. The resulting attachment of the wall to the foundation is a potential point of failure.
Another common fabrication error is oversize holes in the mudsill. The mudsill is the base member of a wall frame that is in direct contact with the foundation. Inaccurate measuring and carelessness in drilling the mud sill during framing of the walls often result in holes in the mud sill which don't line up with the bolts placed in the foundation or in the stem wall. This requires extra holes, or oversize or elongated holes be created in the mud sill which weakens the frame-foundation interface.
The steel attachment hardware that connects the shearwall to the foundation is another point of weakness. If a field fabricated shearwall was ever built in exact compliance with the design, the steel attachment hardware would likely fail before the shearwall. In most cases the steel attachment hardware is fabricated by folding steel strips with a few tack welds. In practice the folds provide the necessary flex in the steel attachment hardware to induce failure. In other cases, the method of attaching the steel attachment hardware to the studs induces cracking of the studs.
Some developers and building departments have been examining the feasibility of using "metal studs" to create the frame of a building. This has the benefit of simpler fabrication. The elements of the wall frame are attached to each other with sheet metal screws or other suitable fasteners, and the metal studs do not split like many boards do when fasteners are driven into them. The metal studs may be cut to exact size with metal shears eliminating the need for dangerous power saws. The metal studs have holes fabricated in them to accommodate electric wiring eliminating the need to drill holes through wooden members wherever wiring is needed. Metal studs alone are not very strong compared to wood, however, metal studs are less expensive than wood. Their advantage is that when the metal studs are used to create a vertical diaphragm or panel by the application of sheathing the resulting panel may support almost as much vertical load as its wooden counterpart at a fraction of the cost.